I. Field of the Invention
This invention relates to a surface light source device. In more detail, the invention relates to a surface light source device using an optical element (hereinafter referred to as "a scattering light conducting element") having a function to conduct an incident light while scattering in a volume region. The invention can be adapted to an optional application requiring a high uniform emitting light flux, and in particular, effectively utilized as a display backlight source means of a liquid crystal display device, etc.
II. Description of the Prior Art
Conventionally, heretofore known are various kinds of surface light source devices of the type capable of emitting the light in a desired direction using the scattering phenomenon, those of which are used for a backlight source of the liquid crystal display devices or the like.
One similar type of such known surface light source devices, includes a light emitting panel in which a light incident means is provided on a lateral side of a light conducting element made of a plate shaped transparent material, a reflection element is provided on a back surface-side, and a light scattering property is given adjacent a front surface-side surface to produce a light emitting surface, and which is used as a backlight of the liquid crystal display device and the like.
Such devices are disclosed in Japanese Patent Laid Opens Sho-62-235905 in 1987, Sho-63-63083 in 1988, Hei-2-13925 in 1990, and Hei-2-245787 in 1990.
Those surface light source devices, spreading in the light emitting direction are produced by irregular reflection or mirror reflection adjacent to the light conducting element or on reflection elements, and the light scattering is not generated in a volume shape from inside of the light conducting means, Therefore, it is theoretically difficult to sufficiently raise a rate of scattering light outputted from the scattering light conducting device.
In the surface light source device, to obtain a uniform irradiation degree, a tilt depending on distance from the light incident means must be given to reflectivity of the reflection element as shown in the known disclosures above. This introduces a complicated larger type structure together with a higher manufacturing cost.
When the surface light source device is used for the backlight of the liquid crystal display devices and the like, there must be sacrificed some of requirements such as brightness, uniformity of irradiation degree, a thinner construction, and economy and the like.
As a second similar type of the known surface light source devices there is used a light diffusing plate in which particle substances having a different refractive index from an extended plate shaped transparent material are diffused into the inside of the transparent material.
The similar types of those are disclosed in Japanese Utility Model Registration Patent Application Laid Open Sho-51-89888 in 1976, Japanese Patent Application Laid Opens Hei-1-172801 in 1989, Hei-1236257 in 1989, Hei-1-269901 in 1989, Hei-2-221925 in 1990, and Hei-4-145485 in 1992.
Japanese Patent Application Laid Opens Hei-2221925 in 1990 and Hei-4-145485 disclose that a light is incident from lateral side of the plate shaped light scattering conducting element, where a reflection element is arranged on surface side of one-side, the other side surface is made a light emitting surface, and this forms a backlight source of the liquid crystal display device and the like.
In these arrangements, a light scattering arises in a volume shaped manner due to irregularity of the refractive index produced by the particle substances scattered and mixed into the inside of the transparent element.
However, various counter measures to uniform illuminance as a surface light source in those surface light source device are employed; namely, to give a gradient to a diffusing concentration of the particle substance which is diffused into the scattering light conducting element; to provide a scattering enforcement means such as mesh shape or dot shape by using light dispersing ink and the like on the back-side of the light scattering conducting element; or in some cases to give a gradient on a density of the mesh shape or the dot shape.
Conventionally, there has been employed a method comprising the step of, raising at most the light scattering power where a scattering power is lowered at a part adjacent to a light source together with the decrease of the scattering power at a portion adjacent to the light source, or raising the light scattering power at most including an enforcement layer of a mesh shape or a dot shape on the backside at a position apart from the light source.
In the background where conventionally the method as described above has been employed, in fact it is an advantage that, to avoid deterioration of illuminance apart from the light source, there must be provided a gradient of the light scattering power in any form depending on a distance from the light source in the case where the light scattering conducting light element is formed by dispersing particles having a different refractive index into the matrix, in addition to a history where it has been considered that generally a scattering enforcement means of a back surface region of the scattering light conducting element is indispensable for maintaining the required amount of scattering light, in the ordinary sized scattering light conducting element.
For another form of technique of the second similar type, it is proposed that a shape of the light scattering conducting element is made a wedge shape or a triangle roof shape, and without providing gradient to a different refractive index substance diffusion density itself within one light scattering conducting element.
For example, Japanese patent Laid Open Hei-4-140783 in 1992 disclosed the surface light source device using a plate shaped member combined by a relationship in that a light scattering conducting element (an opalescent substrate) having a mountain (L character) shaped sectional view is arranged apart from an object to be illuminated, and a transparent substrate having a complementary shape thereto is arranged in the vicinity of a side of the object to be illuminated. A light source is disposed on a lateral side of the plate shaped member. In the surface light source device, a light emitted from the light source travels mainly through the transparent substrate to be incident on the opalescent substrate, where the scattered light travels again through the transparent substrate to be emitted from a surface adjacent a side of the object to be illuminated. As a result, passing two times through a boundary surface between the opalescent substrate and the transparent substrate, this then produces a larger light loss disadvantageously.
A surface light source device has been proposed in which, using the so called injection type polymerization, the light scattering conducting element is formed into one sheet of plate shape by combining two wedge type light scattering conducting elements having different scattering powers which have allowed different refractive index substances to uniformly disperse into each polymer matrix, and thus, a light is incident from the lateral direction (see "PCT/JP92/01230", "Polymer Reprints, Japan Vol. 41, No. 3; 1992", p802, and "Polymer Reprints, Japan Vol. 41, No. 7; 1992", p2945 to 2947).
In the prior art, it is difficult to raise an efficiency for light utilization of the surface light source device in that the light is incident from a lateral direction of a transparent materiel of an extending plate shape and a reflecting element is arranged on one-side surface and a light diffusion property is given adjacent to the other-side front surface. If a rise of illuminance of the light emitting surface is intended, the various enforcement means are more required with the increase of a thickness of the device, producing a disadvantage economically.
In the type using such transparent plate, in the case where a mesh or dot shaped enforcement layer is provided on a backside surface of the transparent plate for enforcing a scattering power and unifying a illuminance distribution, then a drawback arises where the mesh of enforcement layer or the pattern of dots is transparently seen on observing the light scattering conducting element from the light emitting surface-side. In order to prevent this, a light scattering powered film or the like is arranged also on the light emitting surface-side. However, such additional structure causes absorption of the light and produces deterioration of the efficiency for light utilization. Moreover, manufacturing cost is raised with a complicated structure of the device.
On the other hand, combining the-light scattering conducting element with the light source element on which a light is incident from its lateral direction, thereby to produce a surface light source device, and even when employing a means of providing a gradient to a diffusion concentration itself of the particle substance diffused into the light scattering conducting element or of providing the light scattering enforcement layer of the mesh shape or dot shape or the like by using the light diffusion property ink and the like on a back surface-side of the light scattering conducting element, it is impossible to avoid disadvantages, such as the complicated structure of the devices, complicated manufacturing processes, and high grade of the devices and manufacturing processes respectively following the additional technical means.
For a scattering power gradient providing means, in the case of employing a procedure of forming the enforcement layer of mesh shape and dot shape on the light scattering conducting element back surface portion or providing a gradient on the scattering power of the enforcement layer, then a predetermined gradient is required for a distribution density of the mesh or dot patterns, the manufacturing process becomes more complicated, therefore this apparently introduces more disadvantages than the case of forming the simple scattering enforcement layer in view of the manufacturing cost.
In the case where a uniform brightness is intended for the surface light source device by giving a gradient to the diffusion density of a substance having a different refractive index diffused into the polymer matrix, then it is not always easy for industrial technique to manufacture in a rapid and stable manner the light scattering conducting element having a diffusion density gradient as intended, where such is not considered as a technique suitable for mass production.
The technique, which has been proposed by the present inventors, is of a high efficiency of scattering and is capable of relatively freely setting an average scattering power and a scattering gradient.
However as in the Laid Opens and specifications, there arises the same problem as above where the gradient is given to the different refractive index diffusion density of the light scattering conducting element in view of mass production since the product is manufactured by the injection type (casting) polymerization as a wedge shaped light scattering conducting element.
In the surface light source device of the type using the conventional light scattering conducting element, a light amount for emitting from a light output surface is to be secured through making a traveling direction of the light to be random by a light scattering operation given to the inside of the light scattering conducting element, and not rendering a prior condition of the technical problem for allowing the emitting light from the surface light source device to have a particular directionality. Therefore, it is difficult to adjust distribution of a propagation direction of the emitting light from the scattering, light conducting element.
In those circumstances, it is proposed that a directionality of the emitting light of the surface light source device be adjusted by additionally arranging a light emitting direction correction element on a light emitting surface of the light scattering conducting element. However because the light emitting direction correcting element used therein is not a wedge type, the following problems arise.
(1) When combining with the light scattering conducting element of the wedge type, it is difficult to form a whole thereof into a uniform thickness because of a thickness difference between the light incident surface-side and the end portion. PA1 (2) A technical means for adjusting an intensity of the directionality of the light emitted from the surface light source device. There has not yet been proposed an idea in that an intensity of a directionality of the light emitted from the surface light source device is adjusted through an intensity of a scattering power given to the light emitting direction correcting element. PA1 (3) In the case where the known thin sheet shaped light emitting direction correcting element is used for suppressing a thickness of the surface light source device, the manufacture thereof by a simple emitting forming method is difficult. This must be prepared as separated parts by another manufacturing method from the light scattering conducting element. Therefore, it is not advantageous in view of manufacturing cost. PA1 (4) To obtain a light emitting direction correcting function (hereinafter referred to simply as "transverse direction") as viewed from a light incident surface-side of the light scattering conducting element, two sheets of light emitting direction correcting elements are required to be arranged orthogonally with each other. This introduces an increase of the number of parts and a more complicated manufacturing process.